It is well known in the prior art that solid solutions of lead iron tungstate [Pb(Fe.sub.2/3 W.sub.1/3)], often referred to as PFW, and lead iron niobate [Pb(Fe.sub.1/2 Nb.sub.1/2)], often referred to as PFN, have a high dielectric constant when sintered as disks, or as multilayer ceramic capacitors. It is also known that small additions, on the order of 1% or less, of various additives such as manganese oxide, silicon dioxide, zinc oxide, or nickel oxide have been used to reduce the dissipation factor (DF) and increase the insulation resistance of PFN-PFW solid solutions.
For example, U.S. Pat. No. 4,078,938 (Yonezawa et al.) describes compositions consisting essentially of 20 to 50 mole % PFW and 80 to 50% PFN which provide a dielectric constant over 5000 when sintered below 1000.degree. C. as a disk or in a multilayer capacitor. The compositions may also contain small additions of SiO.sub.2, NiO, ZnO, or a Mn-containing compound.
In U.S. Pat. No. 4,544,644 (Yamashita et al.), compositions are described consisting of solid solutions of PFN and PFW with inclusion of 0.5 to 10 mol % of M(Cu.sub.1/2 W.sub.1/2), where M is at least one of Ba and Ca, and optionally 1.0% or less of manganese oxide. Dielectric constants as high as 30,000 were obtained on disc or multilayer capacitors fired between 860 .degree. C. and 950.degree. C.
In U.S. Pat. No. 4,885,267 (Takahara et al.), two kinds of PFN-PFW solid solutions were sintered with a grain growth inhibitor selected from oxides of Dy, Gd, and Sm. High K compositions were obtained with decreased dependence of capacitance on temperature.
In E.P. 376,670 (Kanai et al.), high K compositions are disclosed with low temperature coefficient of capacitance comprising a lead-based perovskite as a major component and an aluminoborosilicate glass containing MgO and BaO. The composition may also contain barium titanate as a major component. Disk capacitors were fired at temperatures between 1000 .degree. C. and 1200.degree. C.
And, U.S. Pat. No. 4,772,985 (Yasumoto et al.) discloses high K thick-film capacitor compositions comprising at least one ferroelectric compound having a perovskite structure and a low melting eutectic inorganic binder.
However, it was found that when PFN-PFW doped with manganese oxide was used as thick-film dielectric on alumina substrates, poor dielectric properties were obtained when the capacitors were processed using a variety of commercial high Ag conductors. It is believed that the inorganic bonding agents in the conductor composition, which are added to achieve adhesion to the alumina substrate, react with the PFN-PFW during the firing process and adversely affect the performance of the dielectric by reducing the dielectric constant (K) and insulation resistance.
It is therefore an object of the invention to provide high K dielectric compositions containing PFN-PFW solid solutions which fire at a temperature below 950.degree. C. and can be used with commercial thick film conductors.